The coherence is used to check the correlation between output spectrum and input spectrum. So you can estimate the power transfer between input and output of a linear system. It shows how well the input and output are related to each other.
Autospectrum
Autospectrum is a function commonly explored both in signal and system analysis. It is computed from the instantaneous (Fourier) spectrum as:
There is a new, fundamental function  cross spectrum  in the dual channel processing. It is computed from instantaneous spectra of both channels. All other functions are computed during postprocessing from the cross spectrum and the two auto spectrums  all functions are the functions of frequency.
Cross spectrum
Based on complex instantaneous spectrum A(f) and B(f), the cross spectrum SAB (from A to B) is defined as:
Amplitude of the cross spectrum SAB is the product of amplitudes, its phase is the difference of both phases (from A to B). Cross spectrum SBA (from B to A) has the same amplitude, but opposite phase. The phase of the cross spectrum is the phase of the system as well.
Both auto spectra and cross spectrum can be defined either as twosided (notation SAA, SBB, SAB, SBA) or as onesided (notation GAA, GBB, GAB, GBA). Onesided spectrum is obtained from the twosided one as:
The cross spectrum itself has little importance, but it is used to compute other functions. Its amplitude GAB indicates the extent to which the two signals correlate as the function of frequency and phase angle <GAB indicates the phase shift between the two signals as the function of frequency. The advantage of the cross spectrum is that influence of noise can be reduced by averaging. That is because the phase angle of the noise spectrum takes random values so that the sum of those several random spectra tends to zero. It can be seen that the measured auto spectrum is a sum of the true auto spectrum and auto spectrum of noise, whilst the measured cross spectrum is equal to the true cross spectrum.
Coherence
Coherence function γ indicates the degree of a linear relationship between two signals as a function of frequency. It is defined by two auto spectra (GAA, GBB) and a cross spectrum (GAB) as:
At each frequency coherence can be taken as a correlation coefficient (squared) which expresses the degree of linear relationship between two variables, where the magnitudes of auto spectra correspond to variances of those two variables and the magnitude of crossspectrum corresponds to covariance.
Coherence value varies from zero to one. Zero means no relationship between the input A and output B, whilst one means a perfectly linear relationship.
There are four possible relationships between input A and output B:
Perfectly linear relationship  Sufficiently linear relationship with a slight scatters caused by noise 


Nonlinear relationship  No relationship 


Low values indicate a weak relation (when the excitation spectrum has gaps at certain frequencies), values close to 1 show a representative measurement.
That means when the transfer function shows a peak, but the coherence is low (red circles in the picture below), it must not necessarily be a real resonance. Maybe the measurement has to be repeated (with different hammer tip), or you can additionally look for the MIF parameter.
Coherence is a Vector channel, and therefore displayed with a 2D graph instrument.
The coherence is calculated separately for each point (e.g. Coherence_3Z/1Z, Coherence_4Z/1Z, …).
In the case of no averaging, coherence is always equal to 1. In the case of averaging and samples GAB influenced by noise, deviations in the phase angles cause that the resulting magnitude GAB is lower than it would be without presence of noise (see the picture below). Presence of nonlinearity has similar influence.